1. Field of the Invention
The invention described herein relates to an apparatus and method for imaging an element within a tissue, for example, imaging a microcalcification within a human breast.
2. Description of the Relevant Art
One of the most common clinical features associated with breast cancer is the presence of microcalcifications. It is often an important sign of early cancer and may be the only abnormality in up to 31% of screening-detected carcinomas. Malignant calcifications often appear in breast as innumerable, irregular salt-like grains. Although the size of calcifications can range from the microscopic up to 2 mm, or more, calcifications typically are between 100 and 500 .mu.m. Important factors in differential diagnosis include the size, shape, number and spatial distribution of calcium particles. Generally, clusters of at least 5-10 microcalcifications must be observed before malignancy is suspected.
Currently, mammography is the most reliable method for imaging calcifications. Other diagnostic techniques, including sonography, thermography, light scanning, and magnetic resonance imaging, cannot adequately image calcium deposits. Because calcification is an important sign of early breast cancer, it can be a useful marker in ultrasonic screening examinations. Ultrasound is not an ionizing radiation and has no known side effects associated with it, relative to X-ray mammography. This makes it particularly attractive for use on women of all ages on a routine basis. Although state-of-the-art mammography is very sensitive in detecting calcifications, it is low in specificity. The relatively high-false positive mammograms result in a low rate of true positive biopsies, and an attendant significant morbidity and high financial cost. For example, it is estimated that out of nearly half a million biopsies performed annually in the US, about 100,000 are done on the basis of microcalcifications detected by screening mammography. The annual cost of these procedures exceeds $250 to $300 million. Of these biopsies, only 24,000 are likely to be an intraductal carcinoma. That is, only one out of every four biopsies proves to be true-positive. A substantial reduction in cost and morbidity can be achieved if the number of negative breast biopsies is reduced. This can be best accomplished by improving the evaluation of microcalcifications.
Clinical evaluation indicates that current ultrasound systems and methods can be prone to high rates of false-positive and false-negative readings, thereby limiting existing ultrasound imaging to use as an adjunct to X-ray mammography. The current methods of ultrasonic imaging use acoustic reflectivity of the tissues to image breasts. Calcium particles are highly reflective compared to the surrounding soft tissues, yet they are difficult to identify reliably on the sonograms. This is largely due to the high background "noise" in the images caused by the coherent interference of the ultrasonic waves.
There have been many attempts to use ultrasonography to image entities within the body. For example, in U.S. Pat. No. 4,867,167, to Magnin (1989), entitled "Method and Apparatus for Determining and Displaying the Absolute Value of Quantitative Backscatter," a selected point in the body is imaged by detecting and utilizing the backscatter attenuation of an ultrasonic signal. In addition to the backscattered signal from body tissues, Magnin's method also requires the detection and analysis of backscatter attenuation from moving blood at a particular point, which is close to the selected point in the body. However, Magnin does not indicate how backscattered signals may be used to image entities independently of moving blood.
Also, in U.S. Pat. No. 5,038,787 to Antich, et al. (1991), entitled "Method and Apparatus for Analyzing Material Properties Using Reflected Ultrasound," the method and device disclosed identify and use critical angles of reflection to evaluate the mechanical properties of a material, for example bone. However, this device is intended to take measurements locally over a selected area, in order to minimize the effects of any non-homogeneity at the site under investigation. Another disclosed device characterizes, small non-homogeneous masses embedded within soft tissue, thus being inadequate for the detection of microcalcifications in the breast.
In U.S. Pat. No. 4,338,984 to Perez-Mendez, et al. (1982), entitled, "Method and Apparatus for Detecting and/or Imaging Clusters of Small Scattering Centers in the Body," a method and apparatus for imaging clusters of calcification in the breast by ultrasound scattering is disclosed. The apparatus includes a plurality of receiving transducers disposed to receive ultrasound energy scattered by inclusions in the breast. The apparatus also employs a water tank or similar coupling means to couple the transmitted ultrasound signals through to the breast tissue. Furthermore, the apparatus and method rely upon phase differences between among the several received scattered energy signals to locate the scattering clusters. The ultrasound frequency used for cluster detection is adapted to generate scattering signals in a manner dependent upon cluster size. This apparatus and method, however, presume that a cluster is a calcification solely on the basis of particle size. Also, the apparatus can experience speckle interference which can lead to erroneous results. In addition, water tanks are very difficult to use for imaging patients. Generally, in such systems there is significant refraction artifact and the motion of breast loosely suspended in water can be a problem.
The advantages of using ultrasound Doppler techniques are described in U.S. Pat. No. 4,770,184 to Greene, et al. (1988), entitled "Ultrasound Doppler Diagnostic System Using Pattern Recognition." This device employs a frequency-analyzed signal from a pulsed Doppler ultrasound examination that is processed using statistical pattern recognition to assess the presence and extent of arterial disease. Although this system can be used to visualize blood vessel wall calcifications and anatomical variations, it requires the flow of blood through the vessel to produce the Doppler signal. However, Greene, et al. do not suggest a system or method by which non-homogeneous entities embedded within other tissues of the body can be located, characterized, and visualized.
In U.S. Pat. No. 5,318,028 to Mitchell, et al. (1994), entitled "High Resolution Ultrasound Mammography System and Boundary Array Scanner Therefor," an ultrasound mammography system that employs a synthetic array and a boundary array concept is described. Although the synthetic array structure reduces the array hardware complexity and resolves projector depth-of-field problems, it requires the synchronization of transmitted pulses to the time between human heartbeats, so that the body motion induced thereby does not corrupt the scanned image. In this system, the reflection data collected from a scan provides a basis for imaging the breast volume scanned. The nature of the tissue is determined by the shape and relative contrast of the image, as determined from the reflectivity characteristics. This system is ostensibly limited to applications where the relative movement between the array and objects in the scanned volume is not excessive during the scan period. Furthermore, the image will also exhibit speckle which, in turn, will make the detection of calcification difficult.
In U.S. Pat. No. 4,771,792 to Seale (1988), entitled "Non-Invasive Determination of Mechanical Characteristics in the Body," a non-invasive system and method for inducing relatively low-frequency vibrations in a selected element of the human body and detecting the nature of responses for determining mechanical characteristics of the element are provided. In this system, a broad-band ultrasonic transducer is employed to impart an acoustic wave into the body at multiple frequencies below 20 kHz. By sensing variations in the time-frequency components of the reflected waveform relative to the transmitted multi-frequency signal, the mechanical characteristics of the element can be determined on the basis of the parameters of vibration and of the components of the vibration mode-shape and mechanical vibrational impedance of the element under study. However, Seale's system and method do not provide high resolution imaging of the tissue surrounding the element, or the element itself, to permit additional analysis.
High-resolution breast sonography presents several technical challenges. For example, breast tissue is heterogeneous, resulting in rapid diminution of the beam, owing to reflection and scattering from the many impedance mismatches of tissue surfaces. Also, extensive refraction from curved breast tissue interfaces adds to ultrasonic beam defocusing. Masses located in the superficial breast tissues may be distorted or missed because they are in the near field of the transducer.
Under existing technology, the heterogeneous intensity pattern of the beam in the near field may cause echoes from surrounding tissues to appear within a cyst, suggesting that it is a solid mass. For this reason, breast examination with a hand-held transducer typically requires either the use of a fluid offset, or a transducer with a built-in fluid offset. For the former, a suitable fluid offset would be a degassed water-filled bag placed between the transducer and the breast, or a commercially-produced water tank or offset device.
Masses deep in the breast may also be distorted owing to the diverging beam that degrades lateral resolution. This distortion may take the form of blurring of the margins of masses, filling-in of shadows behind small masses, and failure to resolve smaller masses. Although phased-array probes can provide high-resolution imaging of breast tissue, phased-array probes typically image superficial masses improperly unless a transducer offset device is used to bring the mass into the slice-thickness focus.
In general, although the high-resolution systems are able to successfully differentiate between benign cysts and solid malignant masses, high-resolution systems are hard-pressed to image calcifications in the breast and are unable to differentiate those that are malignant from those that are benign. See, for example, "Breast Sonography", by Bassett et al., AJR 156:449-455, March 1991; "Sonographic Demonstration and Evaluation of Microcalcifications in Sonography" by Leucht, et al. Teaching Atlas of Breast Ultrasound, 2nd Edition, 1996, pp. 189-204; and Breast Sonography, Diagnostic Ultrasound, Vol. 1, Ch. 25, pp. 541-563, for additional discussion regarding the benefits and drawbacks of using current ultrasound technology for breast microcalcification characterization.
What is needed then is a method and apparatus for providing high-resolution, high-specificity imaging and characterization of an element in a surrounding tissue, for example, for detecting, characterizing, and displaying microcalcifications in a human breast.